1. Technical Field
The invention relates to cable television (CATV) interdiction systems and, more particularly, to a method and apparatus for controlling the jamming parameters in such systems.
2. Description of the Prior Art
In the past, a scrambler has been provided to encode premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/decoder at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the decoder portion of the converter/decoder to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver was particularly expensive. Furthermore, providing a converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols, which in some instances involve multiple layers of encryption by both in-band and out-of-band data transmissions thereby further increasing the costs of the converter/decoder. In addition, scrambling systems may leave artifacts in the final signal.
Consequently, the cable industry has reviewed other technology developed in the early stages of cable television, such as the application of negative and positive traps, and more recent techniques, such as interdiction, to improve CATV systems.
A relatively recent technique for premium channel control is the interdiction system, so called because of the introduction of an interfering signal into a premium channel at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve one or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For the sake of efficiency, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The jamming signal frequency is moved as a function of time from channel to channel. The oscillator is frequency agile and hops from jamming one premium channel frequency to the next. Cable television channels and, of course, premium service channels may extend over a wide range of frequencies, for example, from 54 Mhz. up to and including 1000 Mhz. Thus, if only one oscillator were provided, it would have to be frequency agile over a wide range.
One such system, illustrated in U.S. Pat. No. 4,450,481 by Dickenson, has a single frequency agile oscillator which provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal of approximately the same frequency has been superimposed.
A significantly more advantageous interdiction system is disclosed in U.S. Pat. No. 4,912,760 by West, Jr., et al. which controls a plurality of frequency agile oscillators for each subscriber. Each voltage controlled oscillator is allocated a continuous band of frequencies consistent with the elimination of jamming signal harmonics which could disturb authorized programming at a higher channel frequency. The interdiction apparatus includes generating and storing frequency control words for operating the voltage controlled oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data. In U.S. application Ser. No. 07/476,041, filed Feb. 6, 1990, by West, Jr., et al., the method of programming the time slots of the West I system to vary the dwell time and jamming factor of the jamming signals for different premium channels was disclosed.
In West I and II several voltage controlled oscillators (VCOs) are used to provide jamming signals for a particular subscriber. As is explained in those references each VCO has a limited frequency range because of the efficiency of filtering and noise considerations. The limited range of all the plurality of VCOs when taken together cover the frequency band of interest for the jamming signals. The VCOs are hopped from one frequency to another by a single digital to analog converter (DAC) which is analog multiplexed among them. The DAC outputs an analog signal which is selectively applied to a sample and hold circuit at the input of a particular VCO.
While this configuration has proved efficacious there are some aspects of the system which could be better addressed. The sample and hold circuits present several difficult design issues. If the sample and hold circuits are made inexpensively, then the frequencies of the VCOs will drift during the system dwell time because of the voltage droop of the circuits thereby causing unwanted distortion in the system. It is fairly difficult to make this circuit in a cost effective manner where the frequencies will not drift to undesired extent. One way to use an inexpensive sample and hold circuit while improving its voltage droop characteristic, is to refresh the circuit more frequently with the output of the DAC. However, the refresh rate is also basically the hopping rate of the system which is an important jamming parameter that should remain independently variable. Once the refresh rate has been set, the hopping rate is relatively fixed at that rate or slightly in excess of the minimum refresh rate.
Therefore, it would be extremely advantageous to decouple the interdependence of the refresh rate and the hopping rate to allow the design of a much greater range of hopping frequencies.
Moreover, notwithstanding whether the sample and hold circuits are expensive or inexpensive, or whether they are refreshed quickly or slowly, the circuits are inherently analog and difficult to integrate into a single digital circuit chip. This exacerbates any problems in packaging and miniaturization, and complicates providing the subscriber modules in as large scale integration of an application specific integrated circuit (ASIC).